1. Field of the Invention
The present invention relates to geophysical exploration and logging operations, and in particular to seismic energy sources used in cross hole tomography and inverse vertical seismic profiling.
2. Description of the Related Technology
Geophysical surveying is used to map underground geological formations for determining where possible oil and gas formations may be located. The use of geophysical surveying to determine the best drilling site increases the probability of finding oil and gas in producing quantities. In addition, geophysical surveying may be used in existing wells to evaluate the well's future life expectancy for producing production quantities of oil and gas, or to tell when secondary or tertiary recovery methods are applicable to wells whose production has decreased.
Geophysical surveying uses sound waves to penetrate through rock, sand, liquid and sandstone formations in the earth, and under certain conditions reflect off the rock formations deep in the ground. These sound waves are produced by mechanical, electrical, or chemical sources which produce sound vibrations at frequencies that can propagate through the different layers of rock and sand, to depths encountered in a geophysical survey. The sound waves from the seismic source are received by geophones which are vibration sensors that convert the mechanical vibrations or audio sounds into electrical signals that may be utilized by geophysical survey computer processing systems.
Surface geophysical surveying uses one or more seismic energy sources, such as a thumper, vibrator, dynamite explosion or other means to generate low frequency vibrations of sufficient amplitude to penetrate deep into the ground in order to reach the formations of interest. Geophone receivers are strategically placed at positions located some distance from the seismic energy source and, normally, are used in great numbers. When a seismic energy source is fired, vibration energy travels from the ground surface, either passing thought the various geophysical formations, or when encountering formation boundary layers between geophysical rock and sand formations, are reflected back to the surface. Geophones are placed at the surface around the seismic energy source in a pattern that will capture any energy vibrations reflected from the underground geophysical formations of interest. All vibrational information received by the geophones is recorded by electronic instruments that may include computer signal processing of the information received. Each geophone or array of geophones may be designated by a seismic recording channel and there may be hundreds of channels so recorded during a geophysical survey.
Using a seismic energy source located on the surface may produce overload of the geophone receivers during the initial seismic energy pulse from the surface energy source. On the other hand, the reflected seismic energy pulse, traveling from the surface to deep subterranean geophysical formations and then being reflected back to the surface located geophone receivers, may not have sufficient energy necessary for proper information retrieval. To overcome these weak seismic reflections, the exploration industry may use a geophysical seismic energy source placed deep in the ground in a borehole. This borehole allows the placement of the seismic energy source closer to the geophysical formations of interest. Putting the seismic energy source closer to the formations of interest and farther away from the surface located geophone receivers increases the strength of the reflected vibration signals that may be received by the surface mounted geophones and reduces geophone overload.
Various forms of seismic energy sources may be used downhole, such as air discharge devices, piezo electric transducers or hydraulic thumpers. However, most of the vibrational energy waves are lost in the borehole fluid column directly above and below the seismic energy source. Only seismic energy waves radiating perpendicular to the borehole, in a mostly horizontal plane, are useful in geophysical exploration. The seismic energy waves traveling through the borehole fluid, vertically up and down, serve no useful purpose in obtaining seismic information. Loss of seismic energy waves in the vertical fluid column may be as high as 95%, thus, only 5% of the seismic energy waves may provide useful information. Therefore, energy sources that produce large magnitudes of vibrational waves are necessary in order to obtain readable information by the surface mounted geophones.
Each seismic energy source has a characteristic energy waveform signature that is very important to know when compiling data from a seismic survey. This signature may comprise the energy source frequency, and pulse amplitude and duration. The signature is also dependant on the pressure and density of the surrounding media.
During a seismic survey, at each energy source point in the borehole, a number of recording may be necessary to achieve the desired signal to noise ratio. These multiple records are vertically stacked by correlation techniques. Thus, it is very important that the source signature be identical for most effective vertical stacking of the multiple records.
The second form of geophysical exploration is cross hole tomography which is used to produce a sectional view of geophysical formations located between two or more borehole wells. A seismic energy source is placed in a borehole and geophone seismic receivers are placed in other borehole wells surrounding the seismic energy source. The depth of geophone receivers are varied so as to take a profile of the geophysical formation that exists between the seismic energy source and the geophone receivers. Very little of the reflected seismic waves are utilized in cross hole tomography, primarily the seismic vibrational waves propagated through the geophysical formations are utilized and these vibrations are higher in frequency than vibrational frequencies that would normally be used for deep geophysical surveys using reflected seismic energy waves. When using a seismic energy source in cross hole tomography, the same problem exists as mentioned above in that most of the seismic energy is lost in the vertical fluid column of the borehole, thus, only a fraction of the energy is radiated horizontally into the geological formations of interest.
Present geophysical survey techniques and equipment require vibrational energy sources capable of supplying great magnitudes of vibrational seismic energy to overcome the substantial energy losses in the borehole vertical fluid column. Useful seismic energy from present technology equipment and methods is produced in an inefficient and costly manner because only a small percentage of the generated vibrational energy may be utilized in obtaining useful information from the geological formations.